An important route to the reduction of green gasses lies in energy utilization. In particular, lighting accounts for about 17% of the total energy consumption in buildings. State of the art white organic light emitting devices (WOLEDs) are currently poised to make reductions in this consumption rate for lighting as power efficiencies greater than that of fluorescent lighting have been demonstrated. However, WOLEDs are currently limited by high external quantum efficiencies at low overall brightness and are reliant on precious metal, Pt and Ir, containing phosphorescent dopants. To circumvent these shortcomings, next-generation devices based on nanostructured inorganic phosphorescent dopants and abundant molecular phosphorescent materials are needed.
Near-infrared (IR) luminescent devices are widely employed in various industrial, scientific, laser, telecommunications and medical applications. In recent years there has been a growing interest in infrared organic light emitting devices (OLEDs). The most notable application of these IR-OLEDs is in medical deep tissue imaging, where the transmittance of IR light through body fluid enables the diagnosis of critical life-threatening conditions. Another important application is in telecommunication systems, as IR-OLEDs have low loss optical signal propagation. However, the design of high quantum yield small-band gap emitter molecules has remained elusive. Nonetheless, materials that have been used as IR emitters in OLEDs include rare earth lanthanide ions (Yb, Nd, Er, etc.), organic dyes, transition-metal complexes, low band gap polymers and colloidal quantum dot nanocrystals. However, lanthanides are non-earth abundant; dye-based OLEDs have low efficiency (<0.5%) in the near-infrared and infrared and have issues with device stability; transition-metal-based OLEDs use costly Pt and Ir; and polymer-based OLEDs suffer from low external quantum efficiencies (EQEs) ranging from 0.03-0.05%. Quantum dot IR-OLEDs are solution processible, possess unique size dependent optical properties owing to the quantum confinement effect, and have tunable emissions and device external quantum efficiencies of 2-10%. However, quantum dots contain toxic compounds, such as PbX (where X═S, Se or Te), InAs and HgTe, as well as PbX—CdS and InAs—ZnSe. The European Union and developing countries strictly restrict the use of heavy metals (e.g. Cd, Hg, Pb) in commercial lightning beyond the 100 ppm level. To circumvent these restrictions and growing environmental and health concerns, various non-cadmium based quantum dots have recently attracted the interest of the research community. OLEDs with these materials have comparable performance metrics to their Cd-based counterparts, showing electro luminescence (EL) up to 800 nm. These quantum dots have been made using Si, III-V group elements (e.g. InP), I-III-VI2 group (e.g. CuInS2) and Mn2+ doped ZnS and ZnSe nanocrystals. Moreover, nanocrystals specifically are smaller crystals of the bulk and therefore exhibit crystallographic planes and expose dangling bonds. The nature of the nanocrystal surfaces therefore makes them highly susceptible to degradation through interactions with moisture and oxygen similarly to degradation pathways seen with organic molecules.
Visibly emitting OLEDs have gained tremendous attention since the first demonstration of the 1% efficient bilayer OLED by Tang in 1987. Steady progress in OLED performance has led to the recent commercialization of OLEDs for both lighting and display applications. OLEDs exhibit several advantageous features as compared to traditional lighting and display technologies: 1) each pixel can be color-tuned, and hence can actively generate the desired color instead of relying on white light filtering leading to, 2) nearly infinite contrast ratios, 3) displays with reduced viewing angle dependence, 4) power efficiencies surpassing that of fluorescent lighting, 5) highly color tunable for ‘warm-hue’ lighting, and 6) displays and lighting panels thinner than 1 mm. Advances have also led to phosphorescent OLED lifetimes approaching 106 hours for red and >105 hours for green OLEDs, though lifetime still remains a challenge for phosphorescent organic dopants. The external quantum efficiency of emitted light in the forward viewing direction of an OLED, which relates the number of photons emitted to the number of injected electrons, is:
      η    EQE    =                    γ        R            ⁢              η        S            ⁢              η        OC            ⁢      Φ        =                            η          ICE                ⁢                  η          OC                    =                        q          Ihc                ⁢                  ∫                      λ            ⁢                                                  ⁢                          P              S                        ⁢                          ⅆ              λ                                          where γR is the recombination probability, ηS is the spin formation efficiency, Φ is the luminescence efficiency for the spins produced, ηOC is the out coupling efficiency, ηIQE is the internal quantum efficiency for converting charge into photons, PS(λ) is the measured output spectral power, q is the electronic charge, h is planks constant, c is the speed of light, and I is the electrical current. For fluorescent and phosphorescent devices ηS=0.25 and ηS=1 respectively. Thus, the ability to harvest triplet excitons through efficient phosphorescent emission greatly enhances the overall efficiency potential. For example, phosphorescent devices with ηIQE=100% have routinely been demonstrated for Ir and Pt containing dopants. However, the ability to produce these devices at low cost over a large area without Ir and Pt is still a challenge that must be overcome for the ubiquitous emergence of OLEDs in lighting and other near-infrared applications.